Macrophytic vegetation of the River Swale, Yorkshire

SUMMARY. An account is given of the distribution down the River Swale in summer 1976 of 132 species of photosynthetic plants which are recognizable macroscopically. Some environmental data are included. The river may be regarded as falling into three zones, both with regard to its physical features and its type of vegetation. The upper zone has rocky substrata, fast flows and a macrophytic vegetation largely of bryophytes; the middle zone has unconsolidated substrata and the macrophytic vegetation is relatively sparse; the lower zone is silted and the macrophytic vegetation consists largely of angiosperms. The flora of the Swale is compared with that of the Tees, since eventually the former is likely to receive water from the latter as part of a major water transfer scheme. Eleven species of 'river' angiosperm which are present in the Tees above the projected abstraction point are absent from the Swale above the inflow point.     

Changes in sediment seed‐bank composition of invaded macrophyte communities in a thermal river

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Clonal diversity and spatial genetic structure of Potamogeton pectinatus in managed pond and river populations

Higher levels of genetic diversity of river macrophytes are expected in downstream parts because of potential accumulation of various genotypes from upstream sites. We assessed the clonal diversity and spatial genetic structure of fennel pondweed (Potamogeton pectinatus or Stuckenia pectinata) populations with emphasis on the estimation of dispersal via clonal propagules along a river in connection to upstream ponds. We analysed genetic diversity of 354 plant shoots sampled in 2005 and 2006 at three pond and five river sites in the Woluwe river catchment (Belgium). Nine microsatellite DNA markers revealed 88 genets of which 89% occurred in only one site. Clonal propagule dispersal was detected up to 10 km along the river. Few multilocus genotypes were repeatedly present along a major part of the river indicating vegetative spread. Populations of ponds contained a higher amount of clonal diversity, indicating the importance of local seed recruitment. A fine-scaled spatial genetic structure indicated that most seedling recruitment occurred at a distance <5 m in pond populations whereas clones in river sites were unrelated and showed no spatial autocorrelation. The clonal diversity decreased along the river from upstream to downstream due to establishment of few large clones.     

Seed bank,seed dispersal and vegetation cover: Colonization along a newly‐created river channel

Question : What is the relative importance of the initial seed bank and subsequent seed dispersal for floristic composition of bank vegetation two years after creation of a newly‐cut reach of a river channel? Location : River Cole, West Midlands, United Kingdom. Methods : We took bank and bed sediment samples from a 0.5‐km reach of a new river channel cut into intact flood‐plain. After river diversion, seed samples deposited on artificial turf mats placed on the river banks and flood‐plain edge were taken in summer and winter 2002 and 2003. Seed rain samples from funnel traps were taken during summer 2002 and 2003. We undertook greenhouse germination trials to assess viable seed species within these samples. In summer 2004, we surveyed river bank vegetation. Agglomerative cluster analysis was used to investigate floristic similarity between seed bank, seed rain, seed deposition samples and final bank vegetation cover. DCA was used to explore contrasts between the samples and to assess whether these reflected interpretable environmental gradients. Results : Seed rain samples contained a small subset of species in the summer depositional samples. 38 species were found within the final vegetation, the seed bank, and at least one of the four sets of depositional samples; a further 30 species not present in the seed‐bank samples were present in at least one of the four sets of depositional samples and the final vegetation. Floristic composition of the vegetation was most similar to the depositional samples from winter 2002 and 2003 and summer 2003. DCA axis 1 reflected a time sequence from seed‐bank samples through depositional samples to the final vegetation. Conclusions : Newly cut river banks were colonized rapidly. Seed remobilization and hydrochorous transport from the upstream catchment are important for colonization. Species richness was highest in samples deposited during winter when high river flows can remobilize and transport viable seeds from upstream. This process would also have enhanced the species richness of seed production along the banks during the second summer (2003).     

Wetlands as barriers: effects of vegetated waterways on downstream dispersal of zebra mussels

1. Stream flow is a major vector for zebra mussel spread among inland lakes. Veligers have been found tens to hundreds of km from upstream source lakes in unvegetated stream and river systems. It has been suggested, however, that the downstream transport of zebra mussels is restricted by wetland ecosystems. We hypothesized that vegetated waterways, (i.e. wetland streams) would hinder the downstream dispersal of zebra mussels in connected inland lake systems. 2. Veliger abundance, recruitment and adult mussels were surveyed in four lake‐wetland systems in southeastern Michigan, U.S.A. from May to August 2006. Sampling was conducted downstream of the lakes invaded by zebra mussels, beginning at the upstream edge of aquatic vegetation and continuing downstream through the wetland streams. 3. Veliger abundance decreased rapidly in vegetated waterways compared to previously reported rates of decrease in non‐vegetated streams. Veligers were rarely found more than 1 km downstream from where vegetation began. Newly recruited individuals and adults were extremely rare beyond open water in the wetland systems. 4. Densely vegetated aquatic ecosystems limit the dispersal of zebra mussels downstream from invaded sources. Natural, remediated and constructed wetlands may therefore serve as a protective barrier to help prevent the spread of zebra mussels and other aquatic invasive species to other lakes and ecosystems.     

The Invasion of the Chinese Mitten Crab (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>) in the United Kingdom and Its Comparison to Continental Europe

Owing to its catadromous lifestyle, the Chinese mitten crab, Eriocheir sinensis, allows comparison between a coastal and an inland biological invasion of the same species. Information about the distribution of this species in the United Kingdom has been collected from sightings made by governmental agencies, The Natural History Museum (London) collection, literature, and from the general public. This information indicated that the range of the species has expanded since the species’ arrival in 1973. The spread has been most marked along the east coast northwards to the river Tyne, on the south coast westwards to the river Teign. The expansion range was quantified and compared using geographic information software, and then compared to recorded spread in Europe. Mitten crabs dispersed along the coast at an average rate of 78 km per year (1976–1999), with a recent sharp increase to 448 km per year (1997–1999). These values are comparable with the historic outbreak in continental Europe where the average rate of dispersion along the Baltic Sea coast (1928–1935) was 416 km per year. Comparable figures for the North Sea coast (1923–1954) were 75 km per year with a peak of 168 km per year in 1927–1937. The upstream spread along rivers in the United Kingdom was 16 km per year in 1973–1998 with a marked increase since 1995 to 49 km per year (1995–1998). These data, in combination with population data published for the river Thames, indicate that the population has been increasing since the early 1990s, causing further range expansion into previously uninvaded river systems. The comparison of the spreading behaviour of the ongoing invasion in the United Kingdom with the historic invasion in northern Europe suggests that E. sinensis in future has the potential to establish itself in all major UK estuaries.     

西太湖水生植物时空变化

水生植物在浅水湖泊生态系统中具有十分重要的作用。根据中国科学院太湖湖泊生态系统研究站1989年以来的常规监测资料,将西太湖（除东太湖以外的湖区）划分为9个区,采用点截法（point intercept method）,于2002～2005年对各区水生植物的种类、生物量和空间分布情况进行了6次调查。结果表明：西太湖现有水生植物16种,分属于11科12属;水生植物总面积约10220hm^2,其中沉水植物分布面积约占64．58％;挺水植物约占0．29％;漂浮植物约占38．16％。各个种之间生物量差异显著,马来眼子菜、荇菜、芦苇的生物量在所有水生植物中居前3位。多样性分析表明,水生植物种类4a来未发生明显变化,但种类和生物量季节性差异较大。水生植物呈环状分布在距湖岸5km以内的水域和部分岛屿周围,东岸和南岸为水生植物的主要集中分布区域,分布区连续性好,且水草种类齐全。挺水植物种类单一,仅有芦苇（Phragmites communis）一种,分布区域多限于水深小于1．6m的湖岸;沉水植物共有8种,为水生植物的主要组成部分,马来眼子菜（Potamogeton malaianus）的分布频度最高,在西山岛周围水域逐年扩张,成为该区域的先锋种;漂浮植物3种,主要以荇菜（Nymphoides peltata）为主,在七都水域有逐渐扩张的趋势。马来眼子菜、芦苇、荇菜表现出对水环境较强的适应能力,目前为西太湖的3个优势种。20世纪50年代以来,西太湖水生植物种类减少了50种,其中水质下降是导致水生植物种类不断减少甚至消失的一个重要原因。围网养殖和不合理的捕捞方式也对局部水域的植物造成极大的破坏。水生植物生存环境日益严峻,种群单一化趋势日益明显。     

Long-term studies of the benthic biology of Tees bay and the Tees estuary

Tees bay on the north-east coast of England receives the waters from the highly industrialised Tees estuary. Since 1970 there have been considerable reductions in both the industrial and domestic sewage discharges to the estuary.The benthic populations of Tees bay have been routinely monitored since 1971, grabbing surveys have been completed at six areas in the spring, summer and autumn of each year. In 1979 the survey was extended to include stations within the Tees estuary itself.Tees bay appears to have a stable benthic fauna. Though the simple analysis of faunal statistics has demonstrated fluctuations within the benthos, neither this analysis nor classification analysis has indicated that there were any long-term changes in abundance or diversity.Classification analysis has also failed to differentiate between the fauna of the different areas within Tees bay. However, studies of biomass and T. fabula growth rate did indicate a difference between the areas close to the mouth of the Tees and those remote from it.The studies of the Tees estuary benthos indicated an improvement in both the abundance and the diversity of the fauna. However, the monitoring took place over a limited period and further work is necessary to ensure that this improvement does not simply represent natural fluctuations within the estuarine benthic population.     

Upstream river morphology and riparian land use overrule local restoration effects on ecological status assessment

River restoration is a central issue of present-day River Basin Management. Unfortunately, many studies have shown limited ecological improvements, hypothesizing catchment influences and missing donor populations as main impeding factors. This study evaluates the ecological status after restoration at 46 river reaches in light of catchment influences upstream. Three groups of environmental parameters were investigated: (i) riparian land use and (ii) physical habitat quality in different lengths upstream of the restorations and (iii) land use in the whole catchment upstream. Ecological quality ratios of standardized fish, invertebrate and macrophyte samples were used as response variables. The results imply that sub-catchment variables influence the ecological status more than local habitat improvements. In particular, fish and invertebrate ecological status was positively linked to percent deciduous forest upstream of restored sites, while macrophytes revealed an opposite trend. Furthermore, we found a strong linkage of site-scale ecological status and physical habitat quality up to 5 km upstream of the restorations; the more natural were riparian land use and river habitat quality upstream, the higher was the chance of a good ecological quality in restored reaches. We conclude that site-scale restoration measures are likely to be unsuccessful, if the sub-catchment physical habitat upstream is degraded.     

Introduction and spread of Thiara granifera (Lamarck, 1822) in Martinique, French West Indies

We followed the invasion dynamics of the Oriental thiarid snail Thiara granifera on the Martinique island, French Antilles. This freshwater species was first discovered in 1991 in the Charpentier River, and its spread has since been analysed based on a yearly survey of the malacological fauna at more than 100 sites covering the whole island and representing 50 river systems and three pools. Four river systems were sampled at many sites. Thirteen river systems were colonized by 1997. Colonization within river systems occurred at a speed greater than 1km per year, probably resulting from both active and passive dispersal. Our results can, on the whole, be explained by a simple diffusion process. However, stratified diffusion has to be invoked in at least one river. Moreover, colonization was faster downstream than upstream, suggesting that current velocity plays a significant role in dispersal. Dispersal also occurred between river systems at a mean distance of almost 10km, though with a large variance, in accordance with the scattered colony model of stratified diffusion. The relative frequencies of T. granifera and Melanoides tuberculata, another recent invader of Martinique, were followed at three sites on the Lézarde River. The first species quickly outnumbered the second, though never wiped it out. The data therefore do not support any exclusion phenomena between these two parthenogenetic invaders. Our analysis does not indicate any obvious influence of the rise of T. granifera on the local freshwater fauna.     

Distribution of submersed aquatic macrophytes in the tidal Potomac River

Results of a 3-year survey (1978–1980) and review of historic trends have shown a major decline in the number of species and the distribution of submersed aquatic macrophytes in the tidal Potomac River since the early 1900's. The freshwater tidal river is essentially devoid of plants and only very sparse populations remain in the mesohaline section of the estuary. Present plant populations are largely confined to the transition-zone region where salinity instability at the fresh-to-brackish water interface is believed to reduce biotic stress on submersed vegetation. Many factors may be implicated in the loss of vegetation over major regions of the tidal Potomac River; however, long-term conditions of excessive nutrients appear to be primarily responsible for the present distribution.     

Assessing maximum depth distribution,vegetated area,and production of submerged macrophytes in shallow,turbid coastal lagoons of the southern Baltic Sea

Submerged macrophytes improve water quality in shallow coastal lagoons but eutrophication often resulted in a degradation of macrophytes. Management measures that protect and restore macrophyte stands require knowledge on what limits macrophyte distribution. Information on macrophyte production and distribution in the Darss-Zingst Bodden Chain (southern Baltic Sea) is lacking since an almost complete loss of submerged vegetation in the 1980s. Nutrient input was reduced in the 1990s and macrophytes seem to recover, although turbidity is high and light conditions are still poor. However, this recovery raised hope that returning macrophytes could stabilize sediments and improve water clarity. In this study, seasonal changes in photosynthesis–irradiance curves of selected macrophyte species were used to calculate potential primary production in different depths and turbidity situations. Bathymetry of the area is then used to assess depth distribution and vegetated area. Since the so-calculated depth limits correspond well with the actual depth distribution in the field, macrophyte depth distribution is concluded to be mostly determined by light conditions. Most macrophytes grow in very shallow areas up to 50 cm depth where also 70% of potential primary production takes place. Present light conditions do not support a further expansion of macrophyte distribution in the DZBC.     

Abrupt regime shifts in space and time along rivers and connected lake systems

Regime shifts between clear and turbid water states are commonly found in shallow lakes. These shifts are attributed to a positive feedback between water clarity and submerged macrophytes (underwater plants). Altering the retention time of the water may influence these interactions and thus potentially reduce the probability of alternative stable states. Here we assessed the effect of water retention time on the occurrence of alternative states in water quality of flushed lakes, chains of lakes and rivers using a spatially explicit simple model. Our results indicate that increased flushing of lakes rapidly decreases the range of parameters with alternative stable states up to their total disappearance at a flushing rate of about 50% the algal growth rate. Similarly, in a chain of lakes or in rivers with low flowing velocity, our model predicts that alternative stable states can only occur for systems with a high retention time. Despite the lack of hysteresis at lower water retention times, we predict that abrupt changes between clear and turbid states are still possible both in time and in space. Over a wide range of parameters, the equilibrium state of the chain of lakes shows a steep gradient of vegetation cover. Further, the transient dynamics of the model often include rapid shifts in time. For example, a local regime shift that occurs upstream may propagate through the whole lake chain or river due to a domino effect. All results of the simple model could qualitatively be reproduced with a more mechanistic model. The abrupt rather than gradual response of submerged macrophytes to reduced turbidity levels still makes river systems rather resilient to management measure. The importance of the initial turbidity and the observed domino effect suggest that restoration measures should start upstream and that these measures should eventually trigger regime shifts downstream.     

Behavioral linkage of pelagic prey and littoral predators: microhabitat selection by Daphnia induced by damselfly larvae

Only recently ecologists started treating the previously separately considered benthic, littoral and pelagic zones of lake ecosystems as closely connected compartments. Here we study a link between organisms belonging to a different compartment – namely the pelagic and the littoral – through behavior in a series of laboratory experiments. Waterfleas of the genus Daphnia are inhabitants of the pelagic zone and suffer a high predation pressure from syntopic vertebrate predators (mainly fish). Presumably to escape this predation, they sometimes migrate in the day to the littoral to seek refuge within macrophytes and return to the pelagic at night. Zygopterans from the genus Ischnura do commonly co-occur in ponds with Daphnia and are known as opportunistic predators of Daphnia . In two initial experiments in microcosms in the lab we showed that Ischnura larvae are littoral predators strongly associated with macrophytes. Although we found that predation rates of individual Ischnura larvae on Daphnia are approximately 1.5 fold lower in macrophytes compared to open water, total predation from Ischnura on Daphnia per unit area is tenfold higher within macrophytes than in open water, making the open water a safer place for Daphnia with regard to Ischnura predation. In a third microcosm experiment we monitored horizontal distribution of Daphnia in the absence, presence and odor only of Ischnura larvae. After 2 hours, on average 10% less Daphnia remained within the vegetation when Ischnura larvae or only their odor were present compared to when Ischnura or their odor were absent. We interpret this as a behavioral anti-predation response of Daphnia to the presence of Ischnura larvae that seems primarily chemically mediated. The observed horizontal migration of the pelagic prey driven by the littoral predator may couple both lake compartments and may interact with the predator–prey relationships within the pelagic.     

Impacts of artificial barriers on the connectivity and dispersal of vascular macrophytes in rivers: A critical review

1. Macrophytes play important functional roles in river ecosystems, providing habitat and food, as well as influencing flow, water chemistry, and sediment dynamics. They also represent an important component of river biodiversity.
2. Artificial river barriers have the potential to disrupt macrophyte dispersal, and compromise their distribution and persistence, but little information is available compared to barrier impacts on fish and macroinvertebrates. Here, we review the mechanisms supporting dispersal of river macrophytes in rivers and evaluate the nature of barrier impacts on macrophytes.
3. Hydrochory (dispersal of propagules by water) is the principal mechanism of downstream dispersal, while zoochory (dispersal of propagules by animals) is likely to be the most important vector of upstream dispersal and inter-catchment transport.
4. Most studies have focused on the impact of large structures such as dams, and the findings indicate the impact is highly context dependent. Slow-flowing habitats upstream of dams can act as traps to drifting propagules and thereby interrupt hydrochory. However, the consequences of interrupted hydrochory for downstream populations are unclear. River regulation can result in lower macrophyte diversity, although the lentic habitats associated with reservoirs can also favour an increase in the abundance and richness of macrophyte communities.
5. Instream barriers are unlikely to affect zoochory by birds directly, but barriers are well known to restrict fish movements, so there is considerable potential for barriers to disrupt zoochory by fish, although no empirical study has specifically examined this possibility.
6. There is a paucity of studies examining the impacts of low-head barriers on macrophyte dispersal. Given the influence of macrophytes on river processes, we call for further research into barrier impacts on macrophyte population dynamics in order to gain a better understanding of the consequences of river fragmentation for fluvial communities and ecosystem functioning.

     

Using Macrophytes in Urban Stream Rehabilitation: A Cautionary Tale

Native macrophytes were transplanted into a small urban stream as part of a rehabilitation program, that also meandered the previously channeled stream, naturalized stream banks, and planted native riparian vegetation. Transplanted macrophytes minimized spread of introduced macrophytes and were viewed beneficially by residents, as was the stream rehabilitation. We transplanted the native macrophyte Myriophyllum triphyllum into five larger streams dominated by exotic macrophytes—some of which were weeded prior to transplanting—to see whether Myriophyllum could prevent regrowth of weeded plants. Transplanted Myriophyllum plants were washed away in two streams, reflecting high shear stresses there. Myriophyllum cover in the other streams decreased as weeded plants regrew. Our attempt at eliminating exotic macrophytes in patches in large streams was unsuccessful. Furthermore, council authorities weeded other experimental sections following complaints from residents of excess macrophyte growth. This problem highlighted conflicting multiple values placed on urban streams by managers and the public. A repeat survey of residents living near the original rehabilitated stream showed that many respondents were now critical of excessive plant growth—both in‐stream and riparian. A recurring comment made concerned the apparent lack of maintenance to the stream, giving it an untidy appearance. Difficulties with propagating and transplanting native macrophytes into larger streams, coupled with a negative perception of native vegetation (both in‐stream and riparian) if it looks unmanaged, suggest that planting macrophytes or riparian plants as part of urban stream rehabilitation programs may be more problematic than realized.     

The Meuse river as a corridor for range expansion of the exotic plant species <Emphasis Type="Italic">Sisymbrium austriacum</Emphasis>: evidence for long-distance seed dispersal

Riparian habitats are particularly prone to invasion of non-indigenous plant species and several species have been shown to rapidly expand their range along river networks, possibly mediated by the occurrence of frequent long-distance seed dispersal events. However, there is still relatively little empirical evidence for long-distance seed dispersal along river networks and most studies to date are inconclusive with regards to the direction (upstream vs. downstream) of seed movement. Using assignment analyses based on dominant AFLP markers, we provide empirical evidence that downstream long-distance seed dispersal has facilitated range expansion of the exotic plant Sisymbrium austriacum along the Meuse River. Of 242 sampled individuals, 13 (5.4%) were allocated to a population other than the one from which it was sampled. Of these, nine (3.7%) individuals were assigned to a known population within the area, the furthest being more than 20 km away from the population from which it was sampled. All putative source populations were located upstream, thus providing strong evidence for downstream migration of propagules. These results support the general view that river systems may serve as efficient transport vectors of plant species and thus may play an important role in increasing the spatial spread and range expansion of exotic plant species.     

The role of vegetation in the water budget of the Usangu wetlands, Tanzania

The Usangu wetlands were severely degraded over the last twenty years by cattle and the shortage of water due to rice irrigation upstream. The eastern Usangu wetlands that were previously perennial dried out in 2000 and 2002 in the dry season. Following the removal of cattle in 2006 from the eastern Usangu wetlands, perennial wetlands has re-established itself and in 2011 the vegetation had recovered and covered about 95?% of the wetted surface mainly as floating vegetation. These wetlands are the source of water of the Great Ruaha River and the volume of water entering the river has nearly doubled after cattle removal. We suggest that this may be due to the shading effect of the floating vegetation reducing the loss of water through net evaporation to about 0.5?cm?day^?1 as opposed to about 1?cm?day^?1 for open water evaporation in this tropical climate. This suggests the important role of the biology in controlling the water budget. By contrast cattle and rice farms have not been removed from the western Usangu wetlands, located upstream, where the wetlands are now reduced to small areas fringing the rivers. We suggest that the western Usangu wetlands should also be restored in order to further increase flows in the Great Ruaha River. At the same time water governance in the catchments and irrigation areas upstream of Usangu wetlands is also urgently required because present water yields are insufficient to meet the hydroelectric needs of Tanzania, the water users all along the river, as well as the important coastal wetlands associated with the Rufiji Delta during a drought year.